"Evidence" Exhibition by Chinese Artist Ai Weiwei

A woman walks next to the installation "IOU", made up of certificates of debt, by Chinese artist Ai Weiwei during a media preview of the exhibition 'Evidence' at the Martin-Gropius Bau in Berlin, April 2, 2014.

A woman walks next to the installation "IOU", made up of certificates of debt, by Chinese artist Ai Weiwei during a media preview of the exhibition 'Evidence' at the Martin-Gropius Bau in Berlin, April 2, 2014. (FABRIZIO BENSCH / REUTERS)

A woman walks next to the installation "IOU", made up of certificates of debt, by Chinese artist Ai Weiwei during a media preview of the exhibition 'Evidence' at the Martin-Gropius Bau in Berlin, April 2, 2014.FABRIZIO BENSCH / REUTERS